The world requires ever-increasing amounts of fuel for vehicle propulsion. Means of utilizing fuels more efficiently and with substantially lower carbon dioxide emissions and air pollutants such as NOxs are essential. Vehicles powered by gas turbines can utilize multiple fuels since they are highly fuel flexible and fuel tolerant. In addition, gas turbine engines, because of their lower average operating temperatures compared to piston-based internal combustion engines, can reduce fuel consumption while also reducing carbon dioxide emissions and air pollutants such as NOxs.
The thermal efficiency of gas turbine engines has been steadily improving as the use of new materials and new design tools are being brought to bear on engine design. One of the important advances has been the use of ceramics in various gas turbine engine components which has allowed the use of higher temperature operation and reduced component weight. The efficiency of gas turbine engines can be improved and engine size can be further reduced by increasing the pressure and temperature developed in the combustor while still remaining well below the temperature threshold of significant NOx production. This can be done using a conventional metallic combustor or a thermal reactor to extract energy from the fuel. As combustor temperature and pressure are raised, new requirements are generated in other components, such as the recuperator and compressor-turbine spools.
The use of both metallic and ceramic components in an engine which may have wide variations in operating temperatures, means that special attention be given to the interfaces of the these different materials to preserve the intended component clearances. Control of clearances generally leads to fewer parasitic performance losses. Fewer parasitic performance losses incrementally improves engine efficiency. In addition, the differential expansion of metallic and ceramic components over many thermal cycles can lead to increased wear and degradation of component clearances which, in turn, can result in ceramic component failure either from increasing thermal stresses, crack growth or contact between moving parts.
There remains a need for innovative designs for gas turbine compressor/turbine spools fabricated from a combination of metallic and ceramic materials that maintain a desired control of clearances between various compressor and turbine components. These new designs will allow increased combustor temperatures which, in turn, can improve overall engine efficiency and reduce engine size while maintaining very low levels of NOx production.